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At least seven civilians, including three children, have been killed in Egyptian air strikes in northeast Libya.

The bombings came as Cairo vowed to find those responsible for the beheadings of 21 Egyptian Coptic Christians kidnapped by fighters pledging allegiance to the Islamic State of Iraq and the Levant (ISIL) in Libya's Sirte.

Sources told Al Jazeera on Monday that at least seven people were killed in air strikes in the coastal city of Derna after Egypt's President Abdel Fattah el-Sisi vowed to "punish" those responsible for the beheadings.

Egypt's military said it carried out the raids early on Monday against ISIL camps, training sites and weapons storage areas.

In a statement aired on state television, the military said: "The air strikes hit their [ISIL] targets precisely, and the falcons of our air forces returned safely to their bases."

However, photos published on social media purportedly showed several damaged residential areas in Derna.

Omar al-Hassi, the head of Libya's legally installed government in Tripoli, called the Egyptian raids "terrorism" and denounced them as a "sinful aggression".

"This horrible assault and this terrorism that's been conducted by the Egyptian military represents a violation of sovereignty in Libya and is a clear breach of international law and the UN charter," Hassi said.

Following the raids, Sisi deployed the armed forces to protect key installations and buildings in Egypt.

Fighters pledging allegiance to ISIL released a video on Sunday purporting to show the killing of the Egyptians kidnapped in Libya.

The Egyptian government and the Coptic Church confirmed the authenticity of the footage, which showed the migrant workers, all wearing orange jumpsuits, being beheaded near a waterfront said to be located in the Libyan province of Tripoli.

The men were seized in two attacks in December and January from Sirte in eastern Libya.

Combating ISIL

In the wake of the video release, France and Egypt urged the UN Security Council to meet and consider fresh measures against ISIL.

French President Francois Hollande and Sisi spoke by telephone, highlighting "the importance that the Security Council meet and that the international community take new measures" against the spread of ISIL in Libya.

Egypt later confirmed it had signed a $5.8bn deal to buy French weaponry, including 24 Rafale combat jets, a multi-mission naval frigate and air-to-air missiles.

Libya has slid into chaos after Muammar Gaddafi was overthrown and killed three years ago, as interim authorities failed to confront powerful militias which fought to oust the longtime leader.

Taking advantage of the chaos, ISIL has carried out a string of deadly attacks.

The group has released several propaganda videos showing vows of allegiance from fighters in the country.

In October, Ansar al-Sharia in Derna pledged allegiance to ISIL.

Sunday's video comes less than two weeks after ISIL released a video showing the burning alive of a Jordanian pilot it captured after his plane went down in Syria in December.